1. Field of the Invention
The present invention relates to an X-ray CT apparatus that includes a plurality of X-ray irradiation sources.
2. Description of the Related Art
In an X-ray CT apparatus, an X-ray detector detects a quantity of penetration X-ray irradiated from an X-ray tube, and collects view data based on the X-ray. A tomographic image of a patient is reconstructed based on the collected view data. The view data is a unit of data used for the reconstruction.
In a so-called 3rd generation X-ray CT apparatus, when the X-ray tube and the X-ray detector that is positioned opposite the X-ray tube are rotated around the patient 360 degrees, a limited fan-shaped X-ray is irradiated from the X-ray tube to the patient. The detector that has a plurality of detection elements arranged in an arc shape detects the X-ray penetrated through the patient. At one angle to the patient, a group of the X-ray data detected by the plurality of detection elements is called a view. It is called a scan to collect view data of a plurality of views while the X-ray tube and the X-ray detector are rotated around the patient. Furthermore, the collection cycle of the view data repeated in the scan is called a view cycle. The tomographic image is obtained by applying reconstruction process to the view data of the plurality of views with a high speed reconstruction unit, etc.
For example, when the view data of one view is collected by one degree, the view data of 360 degrees is collected by one scan, which means the number of the view cycles is 360. The image is reconstructed based on the view data of 360 views. The quality of the image improves by increasing the number of the views in the scan.
In a recent X-ray CT apparatus, the view data of 900 views is collected by one scan, for example. That is, the view data of one view is collected every 0.4 degrees, and the view data of 360 degrees is collected. In order to improve the quality of the image, the X-ray CT apparatus which collects the view data of 1200 views or 1800 views by one scan has been proposed. The view cycle is, for example, 0.3 degrees when the view data of 1200 views is collected, and the view cycle is 0.2 degrees when the view data of 1800 views is collected.
Recently, a scan time in which the X-ray tube and the X-ray detector rotate around the patient is 0.5 seconds. There is a desire to shorten the scan time to less than 0.5 seconds in order to observe a moving internal organ, such as a heart, in more detail. However, since much centrifugal force is applied to rotation portions, such as the X-ray tube and the X-ray detector when a rotation speed is fast, it is technically difficult merely to increase a rotation speed in order to shorten the scan time. One proposed solution to collect the view data of the predetermined views for a short time without increasing the rotation speed is set forth in Japanese Patent Publication (Kokai) No. 2002-172112 (the contents of which are herein incorporated by reference), which describes a high speed X-ray CT apparatus that includes a plurality of X-ray tubes and X-ray detectors (see, for example, pages 3-6 and FIG. 1).
However, new problems result from the technique proposed in 2002-172112, such as a large influence of scattered X-rays and shading occurs when the plurality of the X-ray tubes and X-ray detectors are used. The influence of the scattered X-ray is explained with reference to FIG. 1 and FIG. 2.
FIG. 1 shows an X-ray CT apparatus where an X-ray tube 11A is positioned opposite an X-ray detector 16A, and a patient 14 is positioned therebetween. The X-ray detector 16A includes 800 channel detection elements arranged in a line according to the spread of an X-ray path irradiated from the X-ray tube 11A. Regarding one X-ray 13A irradiated from the X-ray tube 11A to the patient 14, a part of the X-ray 13A is absorbed in the patient, and another part of the X-ray 13A penetrated through the patient 14, indicated as X-ray 13B, incidents to a channel 16P of the X-ray detector 16A along an irradiation direction. Slight remaining X-ray 13A scatters all around. A collimator 110 is positioned in front of the X-ray detector 16A according to each channel to prevent the scattered X-ray 13C from being incident to other channels.
However, even with the above-described configuration, the influence of the scattered X-ray is still large. FIG. 2 shows an X-ray CT apparatus where three X-ray tubes 11A, 11B, and 11C are positioned at 120 degree intervals, opposite three X-ray detectors 16A, 16B, and 16C. Each X-ray tube 11A, 11B, and 11C makes a fan-shaped X-ray path 13. In this case, the X-ray detector 16A detects the X-ray irradiated from the X-ray tube 11A, the X-ray detector 16B detects the X-ray irradiated from the X-ray tube 11B, and the X-ray detector 16C detects the X-ray irradiated from the X-ray tube 11C, respectively.
However, the X-ray detector 16A can detect the scattered X-ray 13C irradiated from the X-ray tube 11B and 11C. That is, although the incidence of the scattered X-ray irradiated from the X-ray tube 11A to the X-ray detector 16A can be reduced by the collimator as described with regard to FIG. 1, it is difficult to avoid the scattered X-ray irradiated from the X-ray tubes 11B and 11C to interfere with the X-ray detector 16A. The same is true for the other X-ray detectors 16B and 16C. For this reason, the quality of the image is deteriorated.